1. Field of the Invention
This invention relates to an optical system for making an annular beam, and more particularly to an optical system for making an annular beam which is suitable for use in the optical head of an optical information recording-reproducing apparatus such as an optical disc apparatus.
2. Description of the Prior Art
Heretofore, in the optical head or the like of an optical information recording-reproducing apparatus, it has been desired to condense a light beam into a spot of as small a diameter as possible in order to make the recording density in the recording medium great. Therefore, it has been necessary to use a lens of a great numerical aperture (herein after referred to as N.A.). On the other hand, the lens for such an optical head must be sufficiently aberration-corrected. However, if N.A. is made great a number of lenses become necessary for the correction of aberrations, and this had led to a problem that the weight of the optical head is increased.
So, a method of providing a light-intercepting plate on the central portion of the lens and using only the marginal portion of the lens is proposed in Japanese Laid-open Patent Application No. 116004/1981. However, in this method, part of the light energy emitted from a light source is intercepted, thus resulting in a reduction in the output on the medium. Therefore, instead of the light-intercepting plate, the advent of simple means whereby an annular beam can be obtained without losing energy has been desired.